LAMELLIBRANCHIA TA. 
233 
straight and oblique below, recurving just beneath the hinge-line, and form¬ 
ing a short mucronate extension. 
Test thin, marked by fine concentric striae which are irregularly crowded 
into fascicles, producing an undulated aspect to the surface. The striae are 
sub-lamellose on the anterior side, and sometimes fasciculate on the wing. 
The fascicles on the body are often marked on their summits by sharp, 
regular lamellae. The structure as seen in exfoliated specimens is radiated; 
but this does not appear to be an exterior character. 
The hinge is marked by a single narrow groove; beyond this the charac¬ 
ters of the interior are unknown. 
A large left valve has a length of 45 mm., height 25 mm., hinge-line 
27 mm. A smaller specimen has a length of 28 mm., height 16 mm., 
hinge-line 17 mm. A right valve has a length of 26 mm., height 15 mm., 
and hinge-line 15 mm. 
Formation and localities. In the Chemung group at Philipsburg, Alleghany 
county, N. Y., and in Sullivan township, Tioga county, Pa. 
Leptodesma quadratum, n. sp. 
PLATE XCI, FIG. 15. 
Shell of medium size, rhomboidal or sub-quadrate; body ovate, making an 
angle of nearly 40° with the cardinal line; length one-third greater than the 
height; ante-byssal margin nearly vertical in front, rounding below into the 
shallow sinus, and extending nearly the whole height of the valve; ventral 
margin nearly straight; post-basal margin abruptly rounded and joining the 
wing without limitation. 
Left valve convex, gibbous on the umbo, and obscurely angular along the 
posterior slope. Right valve equally convex. 
Hinge-line straight, equal to three-fourths the length of the valve, and 
equal to the height. 
Beaks situated at the anterior third of the hinge, prominent and arching 
over the cardinal line. Umbonal region gibbous, subtending an acute angle. 
30 . 
